Consumers can interact with merchants to conduct various financial payment transactions. For example, with the proliferation of mobile computing devices (e.g., smart phones and tablet computers, etc.), development of various attachable hardware devices has increased to provide expanded functionality to such devices. One such example is an attachable magnetic card reader device, used for reading credit or debit card information to facilitate the receipt and processing of payments. Some of these reader devices can be physically and communicatively coupled to an associated mobile device using a standard 3.5 mm audio plug when inserted into the headphone port of the mobile device. Given the nature of these devices, it can be desirable to monitor the health of the device's power source (e.g., battery) to determine, for example, an amount of power remaining before the power source is depleted. Such information can be useful, for example, for determining when to power down various components of the device such that the powering down occurs gracefully before the battery power is drained, or before the battery is damaged from overuse.